For the right cavities of a patient's heart, it is generally sufficient to implant endocardial leads through the right peripheral venous network. The implantation of permanent leads in a left heart cavity, however, involves significant operational risks, for example, the passage of bubbles to the vascular network of the brain located downstream of the left ventricle. For this reason, when the left cavity has to be stimulated, most often a lead is not introduced not the cavity to be stimulated, but rather into the coronary system, with the lead having an electrode that is guided to the left ventricle or left atrium and applied against the wall of the epicardium, as appropriate.
A lead of this type is, for example, the Situs LV model, marketed by Sorin CRM S.A.S. (Clamart, France) and described in EP 0993840 A1 and its US counterpart U.S. Pat. No. 6,385,492 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical). Such a lead is introduced through the coronary sinus opening into the right atrium, by an endocardial approach. The lead is then guided and pushed along the coronary vein network to the chosen stimulation site. This intervention is very difficult, given the peculiarities of the venous network and its access paths, including the passage through valves and tortuosities, as well as experiencing a gradual reduction in diameter of the vein as the lead is advanced along the selected coronary vein. Once the target vein is reached, the surgeon must then, first of all, ensure the mechanical stability of the lead into the vein.
Another problem is the difficulty of finding a good stimulation site, to obtain good electrical contact between the stimulating electrode and the tissue of the epicardium, and maintain this contact over time.
In addition, the surgeon must ensure that the chosen stimulation point does not generate phrenic nerve stimulation.
To overcome these difficulties, it was proposed to have multiple electrodes along the lead body to increase the chances of an acceptable compromise, by possibly giving the lead body a particular conformation. The surgeon can choose from among the various electrodes present on the lead body to find the one that provides the best efficiency from the electrical and hemodynamic points of view. One such multiple electrode lead is described in EP 1938861 A1 and its US counterpart US Patent Publication No. 2008/0177343 (both assigned to Sorin CRM S.A.S previously known as ELA Medical). These leads allow in particular to implement the concept of “electronic repositioning” to direct or redirect the electrical field between different electrodes arranged along the pacing lead of the left cavity and/or with an electrode of the pacing lead of the right cavity. The technology allows managing the micro-movements or changes in the hemodynamic behaviour (reverse modeling) simply by reprogramming the generator via telemetry through the skin, without major surgery.
The counterpart of this solution is an increasing complexity of the structure of the lead, an increase of the number of electrodes causing an increase in the number of components, and therefore of electrical connections, or the use of multiplexing circuits for selecting the various electrodes present on a same lead.
US Patent Publication No. 2009/157136 A1 describes a technique of searching for an optimal pacing site using a temporary mapping catheter to be introduced into the coronary sinus. This catheter is a flexible tube open at both ends, and has, optionally, a guide wire. It is equipped with electrically independent multiple distal electrodes, and at its proximal side, a connector for connection to a data acquisition system for identifying the best stimulation site with an algorithm based on cardiac motion.
A permanent conventional multi-electrode lead, having for example a standard diameter of 4.5 to 6 French, is then placed at the selected location, using either the guide wire and a standard over the wire (“OTW”) technique, or the tube, of the temporary catheter.
Another recent development for a left ventricle pacing lead is to reduce the diameter of the implantable part in the coronary system, to a diameter of 4 French (1.33 mm).
The size of the lead body is indeed a factor directly related to the capacity of controlled guiding of the lead in the coronary venous system, so as to select particular stimulation sites located in some specific collateral veins. These sites are reached by means of a vein sub-selection catheter used to place a guiding stylet at the chosen site. Once the vein is selected and the stylet is placed, the surgeon advances the lead body which slides on the stylet, the latter acting as a guide wire of small diameter axially guiding the lead body to the chosen location (an OTW technique).
These solutions however have two notable limitations: (1) The fineness of the lead, whose diameter does not allow to access the deepest collateral veins: for example, the Situs lead referenced above has a diameter of 2.2 mm (6.6 French) and requires a 7 French diameter introducer, and (2) The correct positioning and good maintaining of the electrical contact of the electrode against the tissue to cause stimulation.
The above techniques of multi-electrode leads and electronic repositioning make possible to (more or less appropriately) overcome the second limitation, however they increase the first limitation, to the extent that the multiplication of electrodes and of the internal conductors or components necessarily implies an increase in the diameter of the lead body which reduces its flexibility, making it difficult or even impossible to ensure passage through the tortuous coronary venous system.
The solutions heretofore known are therefore always a compromise between these two constraints.